There is a limit, but I'm not sure exactly what it is. It's pretty big, so the only time I've heard of anyone hitting it was in the Mayflower group, when they were merging a whole lot of duplicates, all of which had bios, and when they were all stuck together it made a really, really big page.
To give you an idea, this is currently the longest page on WikiTree: http://www.wikitree.com/wiki/Carolingian-79
All that fits on the page! And if you're considering making something longer than that, you might want to consider breaking it up somehow. :)